1999 FE600 breaking down at high revs

Hi Guys, Need some advice on possible fix. Rode my 99 FE 600 today, Has new plug, clean left air filter, top filter basically blocked off waiting on replacement, New 98 oct fuel. Kick starts easy without the choke. Idles nicely, runs well when you don't rev it much and use the torque to push you forward. Problem was noticed after riding for a few ks on dirt road. Open it up full throttle and as it get up in the higher end of the rev range is suddenly cuts out, backfires, looses power, momentarily goes again then cuts out, on off etc. When you back off it regains normal running. Pulls like a train in the lower rev range. Plug is black, very sootie and son said every time I opened it up it chucked a heap of black smoke out the back. Not sure if there is any typical issues with this model. It is very much standard. Carb etc It is as if it is hitting a rev limiter but that isn't it. Any help much appreciated.
Pete in Wollongong

My 2006 550 has a single air filter, under the front of the seat. These earlier ones have one in the same position and another one on the left side behind the front tank shroud. Cheers

Thanks gets. I think I will put a kit through the carb as it should not start so easy without the choke. I leaning towards electrical but happy if its just the carb. Will get to the bottom and let you know.

Happy 2022

Pete

It is looking like a new resistor plug coupled with the likely resister plug cap as the main culprit. Put old worn non resistor plug back in and it didn't break down suddenly at the higher revs. still not perfect but an improvement. Cheers Pete